This invention relates to semiconductor devices in general and, in particular, to field-effect semiconductor devices as typified by high electron mobility transistors (HEMTs).
The HEMT of typical prior art design comprises an electron transit layer of undoped gallium nitride (GaN) grown on a substrate of silicon, sapphire or the like via a buffer layer, an electron supply, or barrier, layer of n-doped or undoped aluminum gallium nitride (AlGaN) on the electron transit layer, and a source, drain and gate electrode on the electron supply layer. The AlGaN electron supply layer has a greater bandgap and less lattice constant than the GaN electron transit layer.
Overlying the electron transit layer of a greater lattice constant, the electron supply layer experiences an expansive strain (tensile stress) and so gives rise to piezoelectric depolarization. The AlGaN electron supply layer is also subject to spontaneous depolarization. The piezoelectric and spontaneous depolarizations of the heterojunction between the electron transit layer and electron supply layer create what is known as a two-dimensional electron gas layer in neighboring part of the electron supply layer. The two-dimensional electron gas layer provides a path, usually referred to as a channel, of current flow between the drain and source electrodes. This current flow is controllable by a bias voltage impressed to the gate electrode.
The HEMT of the foregoing general construction was normally on, there having been a source-drain current flow while no voltage was being applied to the gate electrode. It had to be turned off using a negative power supply for causing the gate electrode to gain a negative potential. Use of such a negative power supply made the associated electric circuitry unnecessary complex and expensive. The conventional normally-on HEMT was therefore rather inconvenient of use.
Attempts have been made to devise a HEMT that is normally off. One known approach to that end is by making the AlGaN electron supply layer thinner. A thinner electron supply layer weakens the field of the electron supply layer due to piezoelectric and spontaneous depolarizations, resulting in the diminution of electron concentration in the two-dimensional electron gas layer. The two-dimensional electron gas layer disappears at its part just under the gate when a field due to the potential difference, with no built-in potential or bias voltage, between the electron supply layer and, making Schottky contact therewith, the gate electrode acts upon the two-dimensional electron gas layer of reduced electron concentration. The HEMT can thus be held off between the drain and base electrodes without application of a bias voltage to the gate.
However, the normally-off HEMT based upon this conventional scheme proved to possess the drawback that, by reason of the thin electron supply layer itself, the two-dimensional electron supply layer suffered an unnecessary drop in electron concentration at other than right below the gate, too. The result was an inconveniently high drain-source turn-on resistance.
A solution to this inconvenience is found in Japanese Unexamined Patent Publication No. 2005-183733. It teaches to make the electron supply layer thinner only under the gate by creating a recess in that layer. This solution is unsatisfactory in that the creation of the recess by selective etching of the electron supply layer is likely to lead to the impairment of the crystalline structure of the electron supply layer, as well as that of the electron transit layer, and hence to the deterioration of the electrical characteristics of the HEMT. What is worse, in desired mass production of the normally-off HEMTs, their threshold voltage will fluctuate from one device to another if, as is very likely to occur, their electron supply layers are not etched to an exactly unvarying depth. For these reasons, as far as the applicant is aware, there seem to be no normally-off HEMTs of the above known scheme that are currently available on the market.
Japanese Examined Patent Publication No. 2006-156816 makes a different approach to a normally-off HEMT. It suggests to make the electron supply layer greater in lattice constant than the electron transit layer. Further, in this prior art normally-off HEMT, a piezoelectric layer is placed wholly between the source and drain electrodes on the electron supply (or barrier) layer, and the gate electrode overlies the piezoelectric layer.
In this case a two-dimensional electron gas layer must appear throughout the heterojunction between the electron supply layer and electron transit layer upon voltage application to the gate electrode. This objective makes it necessary that the piezoelectric layer cover the entire spacing between the source and drain electrodes on the surface of the electron supply layer, and that the gate electrode be as large as surface area as feasible. These necessities in turn impose limits on the spacings of the gate electrode from the source and drain electrodes and hence on gate-source and gate-drain antivoltage strengths. Conversely, should the gate electrode be made smaller in size for higher antivoltage strengths, the two-dimensional electron gas layer might not be formed throughout the heterojunction, resulting in a rise in turn-on resistance.